The present invention relates generally to valve switching manifolds and, more particularly, to methods and apparatus for online switching between supply vessels for continuous operations.
Modern methods to continuously manufacture complex photographic products require a constant source of coating solution. These photographic products typically involve the uniform coating of photosensitive compositions on a substrate, such as, for example, a continuous web of paper, cellulose acetate, polyethylene terephthalate, or PEN. Traditionally, a particular coating solution is prepared in one vessel, and coating is performed using that one vessel as the source for that particular coating solution. Coating of that solution from that source continues until the vessel is nearly empty. At that point, the supply to the coating operation is switched to a second vessel, such that the second vessel containing the same coating solution becomes the source for that coating solution. The remaining coating solution in the old or first vessel and the piping associated therewith becomes waste.
In the past, switching between source vessels has been performed using level sensors to sense when the level of coating solution in the vessel is approaching depletion. When using such a method to determine when to switch from one supply vessel to another in a coating operation, there is considerable waste of usable material in the vessel being superseded. Some of the materials used in making photographic coating compositions are very costly and such waste of usable material can represents a great expense.
In current practice, switching may be performed in accordance with method and apparatus disclosed in U.S. Pat. No. 5,156,298 issued Oct. 20, 1992 to LaRue, the relevant disclosure of which is incorporated herein by reference. LaRue provides a change or switching valve at a juncture between the conduits leading from the first and second containers, and further includes a commercially-available conductivity sensor in each of the conduits between the containers and the change valve. A third conduit from the switching valve leads to a coating hopper. The two conductivity sensors are connected by electric leads to a computer. Stored within the computer is a range of values which represents values of the conductivity of composition when it is acceptable for coating. It will be recognized that the conductivity of composition froth is different from the conductivity of composition free of air bubbles, or of air itself, so that it is possible to use conductivity as a metric for determining acceptability or unacceptability of composition for coating. Thus, when the conductivity detected by the sensor goes outside of the range of acceptable values, it can be taken that the composition passing through the sensor is no longer usable for coating. At such time, the computer sends a signal to the switching valve to close off flow from the first vessel and to simultaneously open flow from the second vessel.
It will be recognized that the volume of usable composition, which is wasted each time the switching valve switches from taking supply from one container to taking supply from the other container, is approximately the volume of the length of conduit between the sensor and the switching valve. In some prior art composition delivery systems however, this may still amount to up to several liters of wasted good composition.
Further in the prior art, typically the conduit leading from the second vessel to the switching valve is prepared for introduction of composition from the second container by being back-filled with water from a port in the switching valve to a port in the container valve to purge air from the conduit. Then, and again prior to the actual switching, the container valve is opened and composition is allowed to flow downwards through the conduit, displacing the backfill water through a drain port in the switching valve. Because the composition typically is water-miscible and generally has a specific gravity that is greater than water, there can be considerable mixing of the composition with the backfill water during this downwards purging of water by composition. Thus, an excess of good composition must be diverted to the drain in order to be sure that all the backfill water has been displaced. Otherwise, the first composition sent to the hopper from the second container after switching over will be diluted, resulting in coating defects. Again, several liters of usable material from the second container may be wasted.
It is therefore an object of the present invention to minimize the waste volume of usable coating composition resulting from an online changeover from a first vessel depleted of coating composition to a second vessel containing a fresh supply of coating composition.
It is a further object of the present invention to provide for automatic changeover from a depleted to source vessel to a fresh source vessel.
Another object of the invention is to provide such a changeover without introducing air bubbles into the conduit leading from the switching valve to the coating apparatus.
Yet another object of the present invention is to provide such a changeover between vessels wherein the amount of usable composition wasted by the changeover is substantially zero.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by connecting each of two source vessels to a vessel-switching valve via respective outlet conduits. The vessel-switching valve has a single delivery conduit for delivering coating solution to a coating apparatus. There is a conductivity sensor located in each vessel outlet conduit between a respective vessel isolation valve and the vessel-switching valve. The conductivity sensors are used to determine whether the contents of the outlet conduits are suitable for delivery to the coating apparatus. An acceptable range of conductivity for the composition type is predetermined. At a predetermined time during delivery of the composition from the first vessel to the switching valve, the vessel isolation valve of the second vessel is opened. The then empty outlet conduit from the second vessel is allowed to fill by gravity to the switching valve thereby displacing the air in the outlet conduit upwards by buoyancy through the coating composition in the second vessel. The volume of that portion of the conduit from the vessel first between the conductivity sensor and the switching valve as well as the volumetric flow rate of the liquid composition is provided to a computer or programmable logic controller. Using this information, the computer can calculate the period of time that it will take to exhaust the volume of coating solution in that portion of the outlet conduit. When the sensor in the outlet conduit from the first vessel indicates a conductivity that is outside the predetermined range, the computer begins a timing operation based on the calculated period of time. At the expiration of that period of time substantially the last of the usable coating composition has reached the switch valve. The computer then opens the switch valve controlling flow from the second vessel thereby allowing coating composition to begin flowing from the second vessel. Shortly thereafter, the valve controlling flow from the first vessel is closed thereby shutting off further flow from the outlet conduit from the first vessel and also preventing coating composition from the outlet conduit of the second vessel from backing up into the outlet conduit from the first vessel. Flow is thus changed over from the first vessel to the second vessel. This is accomplished without introduction of any air into the outlet conduit of the second vessel and with substantially no usable coating composition remaining in the outlet conduit from the first vessel. Further, no waste of usable coating composition has been generated in preparing the outlet conduit from the second vessel for delivery of composition to the vessel-switching valve.
This method allows vessels (kettles or any other continuous source of supply) to be switched online with zero liquid waste and without the introduction of bubbles or flow perturbations to coating. A single bubble, 30 microns or larger can cause a coated defect. Flow perturbations as low as xc2x12.0% of aim flow rate can also cause coated waste.